Tach to Pcm

[dayj1]

Originally Posted by aknovaman

Hate to rain on someones parade but your theory on the PCM tach output is incorrect. First the older PCM red/blue actually puts out a positive pulse from ground to about 8 volts. This transition is sufficient to trigger most aftermarket tachometers. The newer ecu don't work the same way. They actually ground the tach output wire for each tach event. The purpose of the pull up resistor is to do that, pull up the voltage to 12 volts between each tach event. Then with each tach event, the pcm output is grounded, thus transitioning the voltage across the external tach threshold. This can be verified using a simple oscilloscope to see the waveform. If you don't have the pull-up resistor connects to 12 volts, the newer PCM output will float anywhere from 1-8 volts erratically. There needs to be a solid/consistent voltage transition across the external tach in order for it to function reliably.

Respectfully, Aknovaman

Based on what you wrote, I believe you and I are in agreement on the operation of the newer ECMs. At least I think we're saying the same thing. I haven't noted the same behavior as you on the older PCMs, but I certainly haven't used/tested every hardware revision. From your description it sounds like some may have an internal pull-up resistor.